The dimensionless operator (REF ) can be presented in terms of the excitonic representation in the usual way.[1]}, [2]} In our case the relevant terms are:
\({\cal H}_{{\rm e\!-\!ph},{\bf q}}={\cal N}_\phi e^{-q^2\!/4}\!\left[{\cal A}_{{\bf q}}\! + (1\!-\!q^2\!/2){\cal B}_{{\bf q}}\right],\) 
